1. Field of the Invention
The present invention relates to an apparatus for recording and/or reproducing information on and/or from a recording medium having a plurality of tracks arranged thereon.
2. Related Background Art
An optical disk has been known as a recording medium. On the optical disk, a plurality of concentric or spiral tracks are formed. Those tracks are divided into a plurality of sectors in order to record variable length data and enhance the accessing speed. The information is recorded and reproduced sector by sector.
In the prior art, an apparatus shown in FIG. 1 has been used to record and/or reproduce information on and/or from the optical disk. In FIG. 1, numeral 1 denotes an optical disk, and numeral 2 denotes an optical system for recording and/or reproducing information by focusing a light beam emitted from a light source (not shown) onto one of the tracks on the optical disk 1. The light beam reflected by the optical disk 1 is received by a tracking error detector 3 and a focusing error detector 4 so that a tracking error signal and a focusing error signal are detected by a known principle. The detected focusing error signal is supplied to a focus actuator 9 which controls the focusing by moving the optical system 2 along an optical axis thereof in accordance with the input focusing error signal. The tracking error signal detected by the tracking error detector 3 is supplied to a tracking actuator 8 through a switch 22. The tracking actuator 8 controls the tracking by moving the optical system 2 perpendicular to the optical axis thereof across the track in accordance with the input tracking error signal.
On the other hand, when the light beam is to be directed to a desired track from the currently irradiating track, the switch 22 is switched from a position a to a position b and a pulse signal is applied from a pulse generator to the tracking actuator 8. As shown in FIG. 2B, the pulse signal comprises a positive-going pulse signal (jumping pulse) for accelerating the optical system 2 toward the desired track and a negative-going pulse signal (braking pulse) for decelerating the optical system 2. The switching of the signal from the jumping pulse to the braking pulse is effected in accordance with a timing signal supplied from a zero-crossing detector 20. The zero-crossing detector 20 generates the timing signal based on the tracking error signal produced by the tracking error detector 3. The tracking error signal varies between a positive value and a negative value with a period corresponding to a track pitch as shown in FIG. 2A as the optical system 2 is moved across the track. The zero-crossing detector 20 detects a point A (zero-crossing point) at which the tracking error signal reaches zero and sends the timing signal to the pulse generator 21, which switches the jumping pulse and the braking pulse at the point A in accordance with the timing signal. At a point B at which the light beam reaches the desired track, the output of the braking pulse is stopped and the movement of the light beam or so-called jump operation is terminated.
However, in the apparatus in which the jumping signal is switched at the zero-crossing point of the tracking error signal, a problem occurs where a method for intermittently detecting the tracking error signal, for example, a sample servo system is used. This is explained below.
In the sample servo system, a servo area is provided for each sector. As shown in FIG. 3A, wobbled pits and a clock pit are performed in each servo area. The clock pit is recorded on the track and the wobbled pits are recorded in front of and behind the clock pit and staggered from the track on both sides thereof.
In such a servo area, when the light beam spot is on the track, an output signal as shown in FIG. 3B is produced and the tracking error signal is zero through the processing of T.sub.A -T.sub.B. As shown in FIG. 3A, where the light spot deviates somewhat from the track toward the wobbled pit A, the AT error signal S is produced through the processing of T.sub.A -T.sub.B as shown in FIG. 3C. When the light spot moves across the track, the value of T.sub.A -T.sub.B changes as shown in FIG. 4.
In this manner, in the sample servo system, the AT error signal is produced discretely for each sector because each sector has the servo byte as shown in FIG. 3A. Accordingly, when the light beam moves across the track, the sampling point having zero value is not always obtained. Thus, in the configuration shown in FIG. 1, the timing signal for commanding the switching of the jump pulse may not be produced.
The above problem is encountered not only in a sample servo system but in other system in which the tracking error signal is intermittently detected. For example, in a method of sampling the signal produced by the tracking error detector at a predetermined period and processing the sampled signals by a digital signal processor (DSP) to produce the tracking error signal, the problem described above is encountered because the error signal is intermittent.